Starting at the top left circle, and moving right, then down one row and moving left, in a snakes and ladders pattern, letters move through the alphabet in steps of 2, 3 and 4, repeating this pattern all the way down.

Working from left to right, letters in corresponding segments of the circles move through the alphabet in steps of 2, 3 and 4, with their relative positions moving one place clockwise at each step.